1. Field of the Invention
This invention relates to lasers, and more particularly to fiber lasers.
2. Description of Related Art
Lasers, particularly those partially or wholly incorporating optical fibers, have emerged as attractive sources of extremely short pulses of light. Fibers lasers are of particular interest because they can be tightly coiled to produce long path lengths in compact geometries. And because fiber lasers can be hard-wired, they can be made impervious to adverse environmental effects—especially when the polarization is fixed in a manner that makes them relatively insensitive to mechanically and/or thermally induced birefringence effects. They can also be a relatively inexpensive, cost-effective solution to the generation of short pulses of light. Rare earth-doped fibers, and in particular neodymium-doped or ytterbium-doped fibers, are particularly advantageous for fiber laser designs because they can be diode pumped and are scalable to high powers. Mode-locked fiber lasers incorporating rare earth-doped fibers have been shown for in several configurations including linear, ring, and figure-eight geometries.
The energy of the pulses generated in fiber laser oscillators is generally limited by effects that cause the pulse to break up into several pulses (called wave-breaking.) Wave-breaking is a consequence of excessive nonlinearity within the oscillator cavity—a limitation that is particularly problematic in ultrashort pulse fiber lasers where the small beam diameter produces high intensities and therefore large nonlinear phase shifts. Prior art work, while descriptive of these propagation effects in fiber amplifiers did not anticipate this regime of operation in a laser oscillator.